Team exploits electromigration to build nanosystems

Portland, Ore. - Chip designers usually take pains to avoid electromigration, an effect that plagues metal layers, especially aluminum. But now, researchers at Lawrence Berkeley National Laboratory have harnessed electromigration down carbon nanotube "pipelines" to deliver a constant stream of indium atoms to nanoelectromechanical systems (NEMS).

"Electromigration is usually a problem area for chip designers, but we have turned it into a useful property," said researcher Chris Regan. "Previously, NEMS achieved atomic-scale accuracy, but only at the expense of requiring manual steps that moved only one atom at a time. Now we have a nanoscale conveyor belt going for the automatic assembly of NEMS."

Regan, a postdoctoral fellow, performed the work under professor Alex Zettl at the University of California at Berkeley. Researchers Shaul Aloni, Ulrich Dahmen and Robert Ritchie at Lawrence Berkeley National Laboratory (www.lbl.gov) assisted in the project.

The researchers now are working to expand the applicability of electromigration to other types of nanoparticles, in hopes of discovering and characterizing the underlying mathematical principles. The group intends to harness those principles, once uncovered, to begin delivering nanoscale materials to their final-assembly sites on future NEMS devices.

Electromigration has been a source of damage in conventional silicon ICs. In areas where current density is very high, electrons can be torn from metal atoms, turning them into positive ions. The interaction between fast-flowing electrons and the positively charged metal ions can rip them physically free, causing displacement that eventually breaks thin metal lines-the most common cause of malfunctions in an aluminum interconnect. Electromigration was one reason for the chip industry's switching to copper interconnects.

"We discovered this transport mechanism accidentally, but now we have switched our whole research direction," said Regan. "We hope to enable NEMS with high-throughput mechanisms that automate moving atoms."

The group's initial work, he said, was "to develop a nanosoldering technique for forming electrical and mechanical joints in situ. We knew that we'd be able to melt the metal particles, and we thought that we might be able to get good joints by carefully controlling the freezing. But the dynamics and possible applications of the process we discovered have been interesting enough to completely reorient our research direction."

The technique promises to improve on current nanostructure tools such as atomic-force and scanning-tunneling microscopes. Those instruments have been useful for moving atoms and molecules around on surfaces, but they are unable to provide a continuous supply of materials.

The nanotube conveyors, driven by electromigration, could become useful in future nanofabrication techniques by delivering controlled amounts of material to a probe system. The researchers envision a scanning-probe manipulator equipped with one of their carbon nanotubes as the probe tip, creating a continuous nanofabrication process.

Particle 'hose'The team discovered that when they applied a voltage potential of as small as 2 V between the ends of a carbon nanotube, indium nanoparticles atop the nanotubes would begin moving along with the current. What's more, when the current was reversed, the indium atoms were returned to their original position.

"It's like a hose-a constant stream of particles," Regan remarked. "And it's reversible: When we change the current's polarity, we drive the indium back to its original position."

To prove the point, the researchers first thermally evaporated a coating of individual indium metal crystals on a bundle of carbon nanotubes. The tungsten tip of a transmission electron microscope (TEM) was then applied to the end of an individual nanotube, and a voltage was applied.

Though the group is still researching the precise mechanisms at work, they have tentatively surmised that thermal energy from the current flow heats and thus dislodges the indium atoms, allowing them to migrate. The atoms are initially found in a series of clusters along the nanotube, and the application of an electric field causes them to migrate from cluster to cluster in one direction. TEM observations clearly showed that the indium particles constantly gave up atoms and traveled in the direction of current flow.

"The indium atoms are moving along the nanotube inchworm fashion, plus their speed can be controlled by how high a voltage is applied," said Regan.

What's more, because the indium atoms are tethered to the nanotube by virtue of surface diffusion, none of the atoms is ever lost. They can be transported up and down the nanotube by applying, then reversing, the voltage, with 100 percent conservation of mass.

Although the group has devised a mathematical model of the process that closely matches observed behavior, there are still some open questions.

The nanostructure properties of the tubes may be playing a critical role, for example. An indium wire will show electromigration toward the anode, whereas indium atoms on a silicon surface will migrate in the opposite direction, toward the cathode. That is the direction of movement found on the carbon nanotubes as well. The finding indicates that different physical structures can produce different electromigration behavior.

Other research groups have been studying the effect, so more surprises might be in store.